A widely used polymeric membrane for use as a proton exchange membrane (PEM) in fuel cells is Nafion®. Nafion is a relatively expensive perfluorinated polymer which has an operating temperature in a fuel cell at approximately 80° C. Above 80° C., however, the membrane loses its effectiveness due to a loss of proton conductivity. The Nafion® membrane technology is well known in the art and is described in U.S. Pat. Nos. 3,282,875 and 4,330,654. Unreinforced Nafion® membranes are used almost exclusively as the ion-exchange membrane in solid polymer electrolyte-fuel cell (SPEFC) applications. The membrane is fabricated from a copolymer of tetrafluoroethylene (TFE) and perfluorovinyl ethersulfonyl fluoride. There are many advantages in operating a PEM fuel cell above 100° C. The advantages obtained when a PEM fuel cell is operated above 100° C. is that the rate of the reaction inside the fuel cell is increased, the catalyst is less susceptible to poisoning by carbon monoxide and the power density of the fuel cell is increased by the reduction of ancillary equipment such as humidifiers and compressors. There are many systems currently being investigated to replace Nafion for use in PEM fuel cells (J. Power Sources, 5044, 1-22, 2002). The most promising of these are the acid-doped polybenzimidazoles (Electrochimica Acta 43, 1289-1294, 1998), sulfonated polyetherketone (Solid State Ionics, 97, 1-15, 1997), and the sulfonated naphthalenic polyimides. However, a majority of the systems under investigation suffer from a lack of mechanical and thermal durability or poor performance. In addition, many of these systems offer no cost performance advantage over the current state-of-the-art.
Lithium based polymer batteries for aerospace applications, for example, need the ability to operate at temperatures ranging from about −70° C. to 70° C. Current state-of-art solid polymer electrolytes (SPE), (based on amorphous polyethylene oxide, PEO) have acceptable ionic conductivities (10E-4 to 10E-3 S/cm) only above 60° C. PEO has moderate lithium conductivity at room temperature (10E-6 S/cm). In addition, it is difficult to process and except for the very high molecular weight polymers not very dimensionally stable. Higher conductivity can be achieved in current PEO systems by the addition of solvents or plasticizers to the solid polymer to improve ion transport. However, these approaches typically compromise dimensional and thermal stability of the electrolyte as well as compatibility with electrode materials. Thus, there is intense interest in developing new electrolytes with acceptable room temperature ionic conductivity without the need for solvents or plasticizers. Some of these new approaches include combinations of polymers (Electrochimica Acta, 43, 1177-1184, 1998), hyperbrached systems (Macromolecules, 29, 3831-3838, 1996), highly ordered Lanmuir-Blogett films (J. Power Sources, 97-98, 641-643, 2001) and polyphosphazenes (Chemistry of Materials, 13, 2231-2233, 2001). All of the aforementioned approaches for lithium battery applications produce electrolytes with a higher ionic conductivity than PEO, but not high enough for future applications and all suffer from poor dimensional stability.
Accordingly, research and development has now focused on the development of proton-exchange membrane fuel cells. In brief, proton-exchange membrane fuel cells have a polymer electrolyte membrane between a positive electrode (cathode) and a negative electrode (anode). The polymer electrolyte membrane is composed of an ion-exchange polymer. It provides for ionic transport and prevents mixing of the molecular forms of the fuel and the oxidant. Solid polymer electrolyte fuel cells (SPEFCs) are a source of quiet, efficient, power. While batteries have reactants within their structure, fuel cells use air and hydrogen to operate. Their fuel efficiency is high, they are quite, operate over a wide power range and are relatively easy to manufacture. For example, during fuel cell operation, hydrogen permeates through the anode and interacts with the catalyst producing electrons and protons. The electrons are conducted by an electrically conductive polymeric membrane through an external circuit to the cathode, while the protons are transferred by an ionic route through the electrolyte membrane to the cathode. Oxygen permeates to the cathode, where it gains electrons and reacts with protons to form water. The products of the SPEFC's reactions are water, electricity and heat.
However, despite their potential, SPEFC have not been commercialized to a large extent due to unresolved technical problems and overall high cost. To make the SPEFC commercially viable, the membranes should operate at elevated temperatures (>120° C.) to increase power density and limit catalyst sensitivity to impurities. Thus, the problems of using solid polymer electrolyte membranes in electrochemical systems, such as fuel cells, at elevated temperatures have not been solved by the electrolyte membranes presently available. Therefore, it is important to develop solid polymer electrolyte membranes that have high proton conductivity, good mechanical strength and long term stability at temperatures above 120° C. and low relative humidity.